The present invention relates to a liquid content detecting apparatus and a method for detecting the content of a liquid component contained in a liquid mixture such as a mixed fuel in a contactless manner.
In recent years, a fuel comprising gasoline mixed with alcohol has become popular for automotive use in many countries including the United States of America, European countries, etc., for the purpose of reducing the consumption of petroleum.
If, however, such an alcohol-mixed fuel is used for engines suited to a gasoline fuel which forms an air fuel mixture having a stoichiometric air/fuel ratio for the proper combustion thereof, the air/fuel ratio of a mixture formed of the alcohol-mixed fuel becomes leaner than that with the gasoline fuel due to the fact that the stoichiometric air/fuel ratio is much lower with a fuel containing alcohol than with a gasoline fuel containing no alcohol. For this reason, the content of alcohol in an alcohol-mixed fuel is detected so that engine control parameters are controlled in accordance with the alcohol content thus detected to properly adjust the air/fuel ratio, fuel injection timing, ignition timing, etc., so as to provide good combustion thereof.
Examples of conventional liquid content detecting apparatuses for detecting the content of alcohol in a mixed fuel based on the refractive index thereof are disclosed in Japanese Utility Model Application Laid-Open No. 62-81064, Japanese Patent Application Laid-Open Nos. 1-262442 and 1-263536, etc. In these examples, however, the temperature dependencies of the refractive indexes of gasoline and alcohol are different from each other, as shown in FIG. 13. Therefore, in order to accurately detect the content of alcohol in an alcohol-mixed fuel, it is necessary to sense the temperature of the fuel and modify the refractive index thereof based on the thus sensed fuel temperature. Thus, the refractive index and the temperature of the fuel are sensed and input to an engine control unit (ECU) in the form of a microcomputer.
FIG. 14 shows the general arrangement of a liquid content detecting apparatus disclosed in Japanese Utility Model Application Laid-Open No. 1-171350. Here, let us assume that a fuel to be detected is an alcohol-mixed fuel comprising a first liquid in the form of alcohol and a second liquid in the form of a gasoline. In FIG. 14, the conventional liquid content detecting apparatus includes a refractive index sensor, which is generally designated by reference numeral 101, for sensing the refractive index of a liquid fuel in a contactless manner, a refractive index calculator 102 for calculating the refractive index of the fuel based on the output signal of the sensor 101, a temperature sensor 103 for sensing the temperature of the fuel in the refractive index sensor 101 and generating a corresponding output signal, and an refractive index modifying means 104 for calculating the content of an alcohol contained in the fuel.
As shown in detail in FIG. 14, the refractive index sensor 101 includes a casing 115 at opposite ends of which a light emitter 111 and a light receiver 113 are disposed in an opposed, face-to-face relation so that light L emitted from the light emitter 111 passes through a cylindrical light guide 112 towards the light receiver 113.
The casing 115 has a hollow interior in the form of a fuel passage 116, an inlet port 118 for introducing a liquid fuel into the fuel passage 116, and an outlet port 119 for discharging the fuel from the fuel passage 116 to the outside. Thus, a fuel enters the casing 115 from the inlet port 118, flows around the cylindrical light guide 112 in the flow passage 116, and exits the casing 115 from the outlet port 119.
The outer peripheral surface of the cylindrical light guide 112 is sealingly supported at its opposite ends by the opposite end walls of the casing 115 through a pair of annular seals 114 which serve to prevent the leakage of fuel from the interior of the casing 115 towards the outside through the outer periphery of the light guide 112 and the opposite end walls of the casing 115.
The refractive index calculator 102 is connected to the light emitter 111 and the light receiver 113 for calculating the refractive index of the fuel in the fuel passage 116 in the casing 115 based on the output signal from the light receiver 113 and generating a corresponding output signal to the refractive index modifying means 104. Specifically, the refractive index calculator 102 calculates the refractive index of the fuel on the basis of a change or difference between the amount of light emitted from the light emitter 111 and that received by the light receiver 113.
The temperature sensor 103 in the form of a thermistor is mounted on the casing 115 for sensing the temperature of the fuel in the fuel passage 116 in the casing 115 and generating a corresponding output signal to the refractive index modifying means 104.
Based on the output signal of the refractive index calculator 102 and the output signal of the temperature sensor 103, the refractive index modifying means 104 calculates the content of an alcohol contained in the fuel in the fuel passage 116.
FIG. 15 shows, in a block diagram, a concrete example of the refractive index modifying means 104. In this figure, a resistor 141 is connected in parallel to the temperature sensor 103, and an operational amplifier 142 has a first negative or inverted input terminal connected to one end of the resistor 141, a second positive or non-inverted input terminal connected to a power supply 143, and an output terminal connected to the other end of the resistor 141. The operational amplifier 142 functions as a non-inverting amplifier in relation to the output voltage Vo of the power supply 143. A resistor 144 has one end thereof connected to the first inverted input terminal of the operational amplifier 142 and the other end thereof connected to ground. An operational amplifier 145 has a first negative or inverted input terminal connected to the output terminal of the operational amplifier 142 through a resistor and to the refractive index calculator 102 through a resistor, and a second positive or non-inverted input terminal connected to ground. An operational amplifier 146 has a first negative or inverted input terminal connected to both the output terminal and the first inverted input terminal of the operational amplifier 145 through respective resistors, a second positive or non-inverted input terminal connected to ground, and an output terminal connected to the first inverted input terminal thereof through a resistor. The operational amplifier 146 generates an output signal VCa representative of the content of alcohol at its output terminal. The operational amplifiers 145, 146 function as inverting amplifiers in relation to signals input to the first inverted input terminals thereof.
FIG. 16 shows the output characteristic of the refractive index calculator 102 in which a refractive index signal VND changes linearly with respect to the refractive index NDf of the alcohol-mixed fuel. Though not illustrated, the refractive index NDf of the fuel is a linear function of the alcohol content Ca and it is thus equivalent to the alcohol content.
FIG. 17 shows the relationships between the temperature T and the resistance RS of the temperature sensor 103 and between the temperature T and the total resistance RP of the temperature sensor 103 including the resistor 141, in which the resistance RS of the temperature sensor 103 alone changes non-linearly with respect to the temperature T thereof, while the total resistance RP of the temperature sensor 103 and the resistor 141 changes substantially linearly with respect to the temperature T thereof.
FIG. 18 shows the relationships between the temperature T and the refractive index NDg of gasoline and between the temperature T and the refractive index NDa of alcohol, in which temperature coefficients .alpha.g, .alpha.a for the refractive indexes NDg, NDa are different from each other.
FIG. 19 shows the relationship of the temperature T and the refractive index Ca of alcohol, which represents a temperature modification error or tolerance in the case where the content of alcohol Ca is calculated based on an alcohol content signal VCa which is modified to a value at a reference temperature To.
The operation of the above-described liquid content detecting apparatus as shown in FIGS. 14 and 15 will be described below with particular reference to FIGS. 16 through 19. First, as shown in FIG. 14, the light emitter 111 emits beams of light L into the cylindrical light guide 112 at a large conical angle, which are refracted at the interface or boundary surface between the fuel, whose refractive index is NDf, in the fuel passage 116 in the casing 115 and the outer peripheral surface of the cylindrical light guide 112, whose refractive index is NDr, at angles of refraction which depend on the angles of incidence of the respective light beams L. Thus, part of the light L from the light emitter 111 is refracted at the boundary surface and enters the body of fuel in the fuel passage 116, whereas the remaining portion of the light L is reflected at the boundary surface back into the interior of the cylindrical light guide 112 and received by the light receiver 113.
In this regard, the critical or minimum angle of incidence, at which the light beams L from the light emitter 111 incident to the boundary surface are totally reflected into the interior of the cylindrical light guide 112, is called the angle of total reflection .theta.r, and there is the following relationship between the angle of total reflection .theta.r and the refractive indexes NDf, NDr of the fuel and the light guide 112: EQU sin .theta.r=NDf/NDr
Therefore, all the light beams L having angles of incidence greater than the angle of total reflection .theta.r are reflected at the boundary surface into the interior of the light guide 112 and received by the light receiver 113.
The refractive index NDf of the alcohol-mixed fuel varies in accordance with the content of alcohol Ca therein, so the angle of total reflection .theta.r accordingly changes with the alcohol content Ca. Thus, the amount of light L received by the light receiver 113 changes in dependence upon the alcohol content Ca in the fuel. For this reason, the light receiver 113 comprises an element such as a phototransistor which generates an electric current having a magnitude proportional to the amount of light L received. The current thus generated is input to the refractive index calculator 102 where it is converted into a corresponding voltage which is proportional to the amount of light L received by the light receiver 113.
Now, let us consider the case in which the fuel to be detected comprises gasoline mixed with an alcohol in the form of methanol; the cylindrical light guide 112 is formed of an optical glass BK7 having a refractive index of 1.52; and the temperature Tf of the fuel sensed by the temperature sensor 103 is room temperature, i.e., 20 .degree. C. In this case, the angle of total reflection .theta.r of gasoline (i.e., a fuel comprising 100% gasoline containing no methanol (MO)), which has a refractive index of about 1.42 at room temperature, is about 69 degrees, whereas that of methanol (i.e., a fuel comprising 100% methanol containing no gasoline (M100)), which has a refractive index of 1.33 at room temperature, is 49 degrees. That is, the higher the alcohol content Ca in gasoline, the lesser the refractive index NDf of the alcohol-mixed fuel and hence the angle of total reflection .theta.r become. Therefore, as the alcohol content Ca in gasoline increases, beams of light L projected from the light emitter 111 at an increasing conical angle of projection can reach the light receiver 113, so the amount of light L received by the light receiver 113 increases. As a result, the output VND of the refractive index calculator 102 decreases in inverse proportion to the increasing refractive index NDf of the fuel, as clearly seen from FIG. 16.
Since the refractive index VD is in inverse proportion to the temperature T, as shown in FIG. 18, the refractive index modifying means 104 modifies the output VND of the refractive index calculator 102 in the following manner.
First, the temperature dependency of the resistance RS of the temperature sensor 103, which is non-linear as shown in FIG. 17, is modified to be substantially linear by connecting the resistor 141 in parallel to the temperature sensor 103, so that the total resistance RP of the temperature sensor 103 and the resistor 141 varies linearly with respect of the temperature thereof. That is, the total resistance RP is expressed by the following equations: ##EQU1## where RS is the resistance of the temperature sensor; R1 is the resistance of the resistor 141; T.degree. is a prescribed reference temperature; RP.degree. is the total resistance of the temperature sensor 103 and the resistor 141 at the reference temperature T.degree.; and .beta. is a temperature coefficient of the total resistance RP.
From equation (1) above, the temperature signal VT output from the operational amplifier 142 is given by the following equation: EQU VT=VO{(1+RP.degree./R2)-{RP.degree..times..beta.(Tf-T.degree.)/R2}(2)
where Vo is the output voltage of the power supply 143; and R2 is the resistance of the resistor 144.
The temperature output signal VT from the operational amplifier 142 is combined with the refractive index signal VND from the refractive index calculator 102 and together fed to the operational amplifier 145 where they are amplified and fed to the operational amplifier 146 which generates a positive output voltage in the form of an alcohol content signal VCa. Here, the refractive index signal VND is expressed as follows: EQU VND=VND.degree.-K.times.NDf.degree.{1-a(TF-T.degree.)} (3)
where VND.degree. is the refractive index of the fuel at the reference temperature T.degree.; K is an output gain of the refractive index calculator 102; and .alpha. is a temperature coefficient of the refractive index NDf.
From equation (3) above, by adjusting the resistance R1 of the resistor 141, the total resistance RP.degree. at the reference temperature T.degree. can be changed in an appropriate manner. Also, from equation (2) above, the temperature coefficient .beta.VT of the temperature signal VT is given as follows: EQU .beta.VT=Vo.times.RP.degree..times..beta./R2
In addition, from equation (3) above, the temperature coefficient .alpha.VND of the refractive index signal VND is given as follows: EQU .alpha.VND =K.times.NDf.degree..times..alpha.
Accordingly, by adjusting the resistance R1 of the resistor 141 and the resistance R2 of the resistor 144, the temperature coefficients .beta.VT, .alpha.VND for an optimal value of the total resistance RP.degree., which is in advance set to be substantially linear, can be made equal to each other. As a result, the temperature coefficient of the alcohol content signal VCa is removed to provide a temperature-compensated refractive index NDf and an alcohol content Ca.
In fact, however, since the temperature coefficient .alpha.g of the refractive index NDg of gasoline is different from that .alpha.a of the refractive index NDa of alcohol, temperature compensation can be made only for one point in the alcohol content. Thus, for example, the alcohol content Ca calculated from the alcohol content signal VCa can be temperature compensated only for an alcohol content of around 50%, and it still has temperature dependency for all other ranges of the alcohol content and does not become constant irrespective of the temperature T.
With the conventional liquid content detecting apparatus a described above, in which the refractive index modifying means 104 comprises the resistor 141 and the operational amplifier 142 both connected to the temperature sensor 103, it is possible to perform temperature compensation only for a certain alcohol content, and an error results due to variations in temperature for other alcohol content ranges. Thus, it is impossible to accurately detect the content of alcohol at all times over the entire temperature range in which the fuel is used.
In addition, the above-mentioned conventional apparatus has another problem. That is, two signals, one in the form of the output signal VND from the refractive index calculator 102 and the other in the form of the output signal Tf from the temperature sensor 103, are required to be input to the refractive index modifying means 104 for modifying the refractive index VND as calculated by the refractive index calculator 102 on the basis of the temperature signal Tf from the temperature sensor 103, so that two signal lines, two connectors and the like are required for these two signals. This results in a complicated wiring arrangement and deterioration in reliability of the device due to increased possibilities of noise interference, failure, etc.